For people who must be on their feet for a long period of time, whether standing still, walking or running, thick soles are extremely useful. Cushion soles are particularly desirable, but some cushion soles have disadvantages of incorrect flexibility, unnatural feelings, or heavy weight.
Some soles which are filled with a pressurized fluid have problems of stability or problems of uniformly filling areas of the sole. In some devices, if a pressurized sole or a pressurized portion of the sole leaks, the sole may become unusable.